That won't work for a group of science nerds. Stationary has no meaning without defining a frame of reference (stationary relative to the planet, stationary relative to the train, etc). The planet itself is moving (which was established in Hero of Ages, so no trying to claim it's a stationary planet that has a sun orbiting it or some crazy thing) and is actually a non-Newtonian reference frame (though for most purposes it could be considered one). On a moving train could also be generally considered a Newtonian reference frame for most of the trip, and from the physics sense is no less valid of a definition of "stationary" than the planet.
I'm really inclined to be a bit annoyed and insist that the reference frame is the planet itself, because that's how it always seemed to work in the books. No idea what that means with respect to Sanderson's future scifi Mistborn, but this seems an acceptable means to maintain part of my sanity for now.
So, Pat's suggestion matches my intuition in that case as well. The open quandry that remains is: the bubble is smaller than the carriage, so one of the following must happen:
1) Part of the carriage (the seats, etc) are inside the bubble, so the whole carriage is Cadmium slowed, even the parts outside the bubble.
2) The parts of the carriage that are inside the bubble are affected, but the parts outside are not. This is probably going to result in an exploding carriage as weird forces act on it.
3) Since parts of the carriage are outside the bubble, none of it is affected. The Cadmium users inside the carriage will perceive the seats approaching them at very high velocity...
1 kind of seems the easiest to me, but probably makes the least sense... I'd assume the bubbles frequently touch the ground, and I don't think the entire planet gets Cadmium slowed.